When comparing iA Writer vs Spacemacs, the Slant community recommends iA Writer for most people. In the question“What are the best distraction-free word processors?”iA Writer is ranked 1st while Spacemacs is ranked 2nd. The most important reason people chose iA Writer is:

Create whole document by including separate images/markdown docs

Pro

Can tell how long it would take to read through what you've written

Pro

A mode that brings focus to the sentence you're working on

Enabling Focus Mode will dim the whole text except the sentence you are currently writing on.

Pro

Markdown support

Pro

Noise Free Writing Experience removes even the program bar

The Noise Free Writing Experience is one of the first approaches to create a first class distraction-free environment. The feature hides the Mac program bar when you start writing.

Pro

Can export a Word document

Pro

Syncs using iCloud and Dropbox

iA Writer can sync documents using iCloud or Dropbox from within the editor for use across multiple devices.

Pro

Available for all Apple devices

iA Writer is available for Mac, iPhone and iPad.

Pro

Preconfigured emacs distro

Spacemacs is just a well-configured Emacs distribution with community-sourced best in class plugins and layers selected to take the setup pain out of Emacs. Evil mode gives the Vim bindings and modes for fast editing, while Helm makes everything discoverable to make learning to be more productive simple and unintrusive.

Pro

VIM Keybindings with EMACS ecosystem

EMACS ecosystem and language support is best in show. The EMACS is a great IDE that was in search of a good text editor. Spacemacs makes EMACS have a good text editor.

Cons

Con

No Windows or Linux support

Con

Emacs is slow

Emacs is single threaded which means that if you enable all the great features you might be used to from Vim, it will run noticeably slower which can be quite frustrating at times. There are efforts at a concurrent Emacs, but they don't seem to be going anywhere.